1. Field of the Invention
The present invention relates to an image formation processing method used in the generation of X-ray tomosynthesis images.
2. Description of the Related Art
As image formation processing methods for digital tomosynthesis using an X-ray diagnostic apparatus or the like, there have been proposed various methods like James T Dobbins III, and Devon J Godfrey, “Digital X-ray tomosynthesis: current state of the art and clinical potential”, Physics in Medicine and Biology Vol. 48, pp. R65-R106, 2003. As a representative technique, a technique called the filtered backprojection (FBP) method like that shown in FIG. 15 is available. The FBP method obtains multiple tomosyntheses by applying a filter to a plurality of images obtained by performing processing such as logarithmic computation for X-ray images captured from a plurality of directions, and performing backprojection for the images. As a filter to be used, for example, a filter called the Shepp-Logan filter (see the equation in FIG. 15) is known. The FBP method is characterized by applying the same filter coefficient to all X-ray images captured from many directions and applying the same filter to each image without changing the coefficient regardless of the position in one image. The filtered backprojection method is essentially a reconstruction method for a CT scanner which can perform perfect reconstruction, and is not an image formation processing method suitable for digital tomosynthesis incapable of perfect reconstruction. Owing to the characteristic that the processing time is short, however, there are many reports of research on the application of the filtered backprojection method to digital tomosynthesis.
In applying the filtered backprojection method to digital tomosynthesis, several image formation processing methods capable of obtaining high image quality have been proposed. For example, the result obtained by comparing an iterative image formation processing method called Maximum Likelihood algorithm (ML method) and the filtered backprojection method has been reported in Tao Wu, Richard H. Moore, Elizabeth A. Rafferty, and Daniel B. Kopans, “A comparison of reconstruction algorithms for breast tomosynthesis”, Medical Physics, Vol. 31, No. 9, September 2004. This report indicates that the ML method is capable of obtaining better image quality than the filtered backprojection method, but the processing time in the ML method is three hr or more in contrast to the filtered backprojection method in which the processing time is 15 min or less. In addition, an image formation processing method called matrix inversion tomosynthesis disclosed in, for example, U.S. Pat. No. 4,903,204 is known. According to this technique, Ying Chen, Joseph Y. Lo, James T. Dobbins III, “Noise power spectrum analysis for several digital breast tomosynthesis reconstruction algorithms”, Proc. SPIE vol. 6142, pp. 1677-1684, 2006 has reported that image quality higher than that obtained by the filtered backprojection method can be obtained.
In a clinical case, when X-ray tomosynthesis images are to be generated for examination, it is essential to provide image quality high enough to allow interpretation to detect the presence/absence or state of an abnormality. In practice, providing an examination image within a short processing time, e.g., several min, is as important as providing high image quality.
As described above, however, the conventional filtered backprojection method suffers from poor image quality, although the processing time is short. In addition, other image formation processing methods which provide high image quality take a long processing time, and hence are not good enough for practical use.